The present invention relates to a method of manufacturing a glazing panel comprising sheets which are joined together along the margin of the panel using heat-activatable bonding medium which is electrically conductive and/or in contact with electrically conductive material and which is activated in situ by induction heating.
Such a method is applicable for example in the manufacture of hollow glazing panels, the sheets being bonded together by intervening spacing means. The spacing means may for example comprise a metal spacer rail or rails which is or are bonded to metallised margins of the sheets by solder which is melted in situ. As an alternative a heat-activatable adhesive composition can be used for bonding the sheets to a spacer of metal, glass or other material. As a further alternative the spacing means may be constituted by the heat-activatable bonding material itself.
Various proposals to join assembled components of a hollow glazing panel by using an induction heating step are described in literature, e.g. in British patent specifications Nos. 831 166, 1 307 843 and 1 506 282. Most of the prior proposals are of a general nature in the sense that they refer to induction heating as one of the possible ways in which jointing material can be heated in situ, but give at best very little information concerning the form of induction heating apparatus and the procedures which should be used.
In the above mentioned patent specifications: British Pat. No. 831 166 simply states that the assembled components, in that case glass panes and an intervening copper spacer strip, can be placed on a conveyor, moved into a tunnel oven wherein the work assembly is raised to 500.degree. C. and then moved past an alternating magnetic field whereby the temperature of the spacer strip is raised by the induced current sufficiently to fuse the edges of the ring to the glass panes. In this method the heating is sufficient to melt the portions of glass which are in contact with the metal ring so that no separate bonding medium is needed, but the specification does indicate that the metal can be coated with a layer of a bonding agent such as easy-melting powdered glass or borax, in order to improve the wetting of the metal by the molten glass.
British patent specification No. 1 307 843 states that bonding medium for bonding the glass panels of a double glazing panel to an intervening metal spacer can be activated in situ by subjecting the assembly to an electrical heating treatment such as induction or resistance heating; but it does not give any information concerning suitable electrical heating apparatus or procedures.
British patent specification No. 1 506 282, which likewise refers to heating of the spacer rail or rails of a double glazing panel by means of an inductive eddy current, does include an outline of possible procedures. The specification says that the spacer rail or rails can be heated as a whole by means of inductive eddy current and goes on to state that satisfactory results may be achieved in many cases if a relatively large portion of the spacer rail is gradually heated by means of induced eddy currents to the temperature necessary for the joint sealing and the heat is thereafter allowed to progress successively and gradually along the spacer rail, e.g. by a slow successive relative displacement of the eddy current source with respect to the spacer rail in the longitudinal direction. In a specific embodiment use is made of high-frequency coils and a longitudinal portion of the spacer rail corresponding substantially to the diameter of the high-frequency field is slowly heated to the jointing temperature before the panel assembly is displaced to conduct its adjacent edge areas successively through such field.
When assessing the suitability of an inductive heating method for use in the production of panel joints under industrial mass production conditions, various factors need to be considered. Most important of course is the quality of the panel joints and the reliability with which a given joint standard can be reproduced. The panel joints must not only have a certain minimum strength to withstand forces imposed on the panel in use, but they should be of uniform quality around the panel.
The formation of joints satisfying a given quality standard is dependent on the generation of an appropriate amount of heat in the heat-activatable bonding medium and usually both the temperature to which the bonding medium is raised and the heating time must be within certain limits. For example, when manufacturing glazing panels in which metallised margins of the glass sheets are soldered to an intervening metal spacer, it is important for the solder to be sufficiently heated to become molten to give good wetting of the metallised sheet margins and the spacer and to produce well-formed solder beads but the molten state must not persist for more than a very short time otherwise there would be a risk of corroding the contacting metal, particularly the said metallised sheet margins.
The heating effect of an induction heating apparatus operated at a given inductor input power depends on a number of factors including the composition of the work to be heated and the dimensions thereof, and also to its spacing from the inductor. An appreciable amount of experimentation may be required to establish appropriate settings of the apparatus for particular circumstances. The control of the heating apparatus for jointing different panel assemblies, and particularly for jointing panel assemblies of different dimensions, e.g. different thickness and/or length and breadth dimensions, therefore involves considerable difficulty.